Light sources for interferometers need to meet certain requirements in terms of temporal coherence and spatial coherence for various applications, while outputting sufficient power. Before laser devices, atomic spectrum lamps such as sodium lamps and mercury lamps were generally used as the light sources for the interferometers. Such light sources emit radiations with spectral components at discontinuous and discrete wavelengths. To meet the requirements on temporal coherence and spatial coherence, it is common to filter out unwanted spectral components with a filter sheet and to limit a light source in dimension with an aperture following the light source. However, the atomic spectrum lamps are generally extended light sources. Thus, the aperture following the light source will significantly reduce effective energy entering the interferometer from the light source, so that it is difficult to observe bright interference fringes on an observing screen. This prevents accurate measurements by the interferometer. Such problems are completely solved by the laser devices. The laser has excellent temporal coherence and spatial coherence, and thus has a coherence length that other types of light sources cannot achieve. Due to the excellent spatial coherence, the laser has a broad coherent region in space, implying that limitation by an aperture following the light source is not needed any more. The laser has highly concentrated energy density, resulting in sufficiently bright interference fringes. Therefore, the laser light sources have become most desirable ones for the interferometry.
In laser interferometers, coherent noises will influence the accuracy of data processing on interference patterns. The coherent noises can be caused by undesired reflection, diffraction, and scattering. The diffraction may result from dust particles, rough surfaces, and scratched surfaces. Because the He—Ne laser light source has excellent coherence, interference fringes detected by a CCD comprises not only those due to interference between a reference beam and a test beam, but also stray fringes due to interference between stray light beams reflected by respective optical surfaces in the light path and the reference and test beams, resulting in a degraded interference pattern and a reduced system transfer function. To address the issue, sometimes low-coherence light sources or white light sources are used for the interferometers, or a rotating ground glass may be incorporated into an imaging system, to reduce the spatial coherence. However, the spatially extended light sources will significantly reduce the contrast of the interference fringes and the signal-to-noise ratio of the interferometers.